1. Field of the Invention
The present invention relates to a television camera apparatus using a solid-state image pickup device having a potential well and, more particularly, to a solid-state image pickup apparatus which can prevent deterioration of an image quality.
2. Description of the Prior Art
In the case where a television camera apparatus using a charge coupled device (CCD) is used under circumstances in which a dark current increases, such as, for example, under high temperature conditions, a picture quality can deteriorate greatly due to both such an increase in the dark current and an increase in variation of the dark current for every pixel, which is caused by the difference of the characteristics of respective pixels constituting the solid-state image pickup device, and at the same time the S/N ratios of luminance and chrominance are degraded to considerable degree.
Such deterioration will be described with regard to an example of the solid-state image pickup device which is operated by way of a single phase driving system, with reference to FIGS. 1 to 5.
FIG. 1 is a diagrammatical view of one pixel of a solid-state image pickup device. FIG. 2 is a waveform diagram of driving voltages for driving the solid-state image pickup device in the interlacing mode or non-interlacing mode. FIG. 3 is a diagram showing the states of the potential wells of virtual phase shift sections 4 and 4' and of a driving phase shift section 3 in one pixel when a middle voltage V.sub.M is applied to a transfer electrode of the solid-state image pickup device. In the virtual phase shift sections 4 and 4', the potential wells are set to a plurality of potential levels in a stepwise manner by changing the concentration of P ions. FIG. 4 is a diagram showing the states of the potential wells of the virtual phase shift sections 4 and 4' and of the driving phase shift section 3 in each pixel when a driving voltage V.sub.0 is applied to the transfer electrode of the solid-state image pickup device. FIG. 5 is a diagram showing an increase in dark current for a change in temperature of the solid-state image pickup device.
In FIG. 1, reference numeral 1 denotes a semiconductor substrate consisting of P-type silicon or the like; 2 is a transfer electrode consisting of transparent polysilicone having conductivity or the like; 2' is a lead connected to a drive circuit for driving the solid-state image pickup device; 3 a driving phase shift section; 4 and 4' virtual phase shift sections; 5 a silicon oxide film having an insulating property; and 6 a shielding layer. In the diagram, a dope portion of n ions is indicated by a mark (+) and a dope portion of p ions is represented by a mark (-) in the shielding layer 6.
The dope portion of p ions existing near the surface of the virtual phase shift portion 4 is connected to a channel stopper (not shown), so that the surface potential is fixed to the potential of the semiconductor substrate 1. The surface of the driving phase shift portion 3 faces the transfer electrode 2 through the silicon oxide film 5. The voltage V.sub.1 =K.sub.1 V.sub.D +K.sub.2 V.sub.SUB which is derived by dividing and adding a transfer electrode voltage V.sub.D and a voltage V.sub.SUB of the substrate 1 is applied to the surface of the driving phase shift portion 3, wherein K.sub.1 and K.sub.2 potential constants which are determined by the thicknesses and dielectric constarts of the film 5 and substrate 1.
FIG. 2 shows signal waveforms of the driving voltages for making the virtual phase shift portion 4 operative. In the diagram, the signal charges generated in correspondence to a quantity of light of an object to be photographed which enters the photo sensitive portion of the solid-state image pickup device are accumulated for an accumulation period of time indicated at t.sub.1. The signal charges are transferred to the accumulating portion of the solid-state image pickup device for a transfer period of time t.sub.2. The middle voltage V.sub.M is applied to the transfer electrode 2 to make the solid-state image pickup device operative in the interlacing mode. The driving voltage V.sub.0 is applied to the transfer electrode 2 to make the solid-state image pickup device operative in the non-interlacing mode. Reference characters A and B represent voltage levels in operation in the interlacing mode and non-interlacing mode, respectively.
The interlacing mode in which the signal charges of each pixel are taken out by performing the interlacing scanning will be described with reference to FIGS. 3 and 2.
As shown in FIG. 2, the driving voltage V.sub.D which is applied to the transfer electrode 2 is maintained to the middle voltage V.sub.M for the accumulation period t.sub.1 in the interlacing mode. In this case, as shown in FIG. 3, the potentials 7 of the virtual phase shift portion 4 and of the driving phase shift portion 3 are the same level, so that the photoelectrons e generated by the incident light .nu. are accumulated in wells 8 in the virtual phase shift portion 4 and driving phase shift portion 3 by the same quantity, respectively.
As shown in FIG. 2, for the transfer period t.sub.2, the level of the first driving voltage which is applied to the transfer electrode at the start of the transfer period t.sub.2 is switched in dependence on the odd field (Odd) and even field (Even). Namely, as shown in FIG. 2, the potential is dropped at the start of the transfer of the signal charges in the odd field and the potential is raised in the odd field. Due to such increase and decrease in the potential, as shown in FIG. 3, in the odd field, the signal charges accumulated in the well 8 of the driving phase shift portion 3 are added in the well 8 of the virtual phase shift portion 4' to the signal charges accumulated in the well 8 of the portion 4'. Thus, the center of sensitivity exists at the middle position between the portions 3 and 4'. In the even field, the signal charges accumulated in the virtual phase shift portion 4 are added in the driving phase shift portion 3 to the signal charges accumulated in the portion 3. Thus, the center of sensitivity exists at the middle position between the portions 3 and 4'. Then, the pulse driving voltage having the levels of the zero potential and of the driving voltage V.sub.0 as shown in FIG. 2 is applied to the transfer electrode 2, thereby transferring the signal charges accumulated in each well 8.
The non-interlacing mode in which the interlacing scanning is not carried out will now be described with reference to FIGS. 4 and 2.
As shown in FIG. 2, the driving voltage which is applied to the transfer electrode 2 is maintained to the driving voltage V.sub.0 for the accumulation period t.sub.1 in the non-interlacing mode. In this state, potentials 7 and 7' of the driving phase shift portion 3 and of the virtual phase shift portions 4 and 4' differ. Therefore, all of the photoelectrons e generated by the incident light .nu. are collected to the portions 4 and 4' and the signal charges accumulated therein are transferred by applying to the transfer electrode 2 the pulse driving voltage having the levels of the zero voltage and of the voltage V.sub.0 as shown in FIG. 2.
As described above, the solid-state image pickup device can perform any operation in the interlacing mode or non-interlacing mode in correspondence to the level of the voltage applied to the transfer electrode 2.
Referring now to FIG. 3 showing the operation in the interlacing mode and FIG. 4 showing the operation in the non-interlacing mode, the potential distributions 7 and 7' near the surface of the driving phase shift portion 3 and the states near the surface thereof differ as indicated by the presence and absence of the wells 8. Practically speaking, as shown in FIG. 3, when the voltage of the middle level V.sub.M is applied, the portion near the surface of the driving phase shift portion 3 becomes the depletion portion. However, when the voltage V.sub.0 is applied, as shown in FIG. 4, the portion near the surface is filled with holes indicated by a mark (X). Namely, this is because the driving voltage V.sub.0 applied to the transfer electrode 2 exceeds the pinch-off voltage, so that the holes are implanted into the portion near the surface of the driving phase shift portion 3 from a channel stopper (not shown) and its potential is held to zero.
Therefore, the magnitudes of the dark currents produced due to generation of the electron-hole pairs due to thermal excitation differ in the interlacing mode and non-interlacing mode. Namely, thermoelectrons generated in the portion near the semiconductor substrate 1, which becomes a main cause of an increase in dark current, are combined with the holes existing near the surface of the driving phase shift portion 3 in the non-interlacing mode. Thus, only the thermoelectrons generated in the wells of the dark current are accumulated in the wells.
On the contrary, in the interlacing mode, since no holes exist in the portion near the surface of the driving phase shift portion 3 in FIG. 3 as mentioned above, the thermoelectrons generated in the portion near the surface also enter the wells and the dark current increases largely. Such an increase in dark current results in increase in variation of the dark current.
The dark current which increases in response to a change in temperature of the solid-state image pickup device will then be described with reference to FIG. 5.
In FIG. 5, the abscissa indicates the driving voltage V applied to the transfer electrode and the ordinate represents the dark current and reference characters T.sub.0, T.sub.1 and T.sub.2 show temperatures of the solid-state image pickup device. There is the relation of T.sub.0 &lt;T.sub.1 &lt;T.sub.2 among those temperatures. When the driving voltage V.sub.D shown in FIG. 2 is lower than the pinch-off voltage V.sub.P (example, when the driving voltage is V.sub.0), the holes are implanted into the portion near the surface of the driving phase shift portion 3 as mentioned above, so that the dark current is very small. However, when a driving voltage is larger than the pinch-off voltage V.sub.P (e.g., when the voltage V.sub.M is applied), the dark current increases. Further, the dark current increases more and more with an increase in temperature. On one hand, this dark current also increases depending on the accumulation period of time.
As described above, the solid-state image pickup device can perform both operations in the interlacing and non-interlacing modes in dependence on the difference in driving voltage applied to the transfer electrode. However, in the case of executing the operation in the interlacing mode, the dark current increases and the variation of the dark current becomes large as compared with those in the operation in the non-interlacing mode.
Therefore, in particular, when the interlacing operation is carried out or when the temperature of the solid-state image pickup device is high and the accumulation time is long, the black level of the video signal fluctuates due to the increase in the dark current and the variation of the dark current. Thus, the balance of black on the screen becomes unbalanced and the picture quality deteriorates.